Hydrocracking heavy petroleum-based hydrocarbon feedstocks into lower molecular weight products such as liquid petroleum gas, gasoline, jet fuel and diesel oil is well known in the art. In recent years, the processing of vacuum gas oils (VGO) into high-quality middle distillates has become increasingly important as crude quality has fallen and the demand for cleaner burning diesel and jet fuel has increased.
To enhance the quality of a refinery product slate (as well as product selectivity and flexibility to meet new market demands), it is a common practice to hydrocrack a feedstock, such as VGO, at either a relatively low or high pressure and then introduce the hydrocracked effluent as a partially converted, high quality feedstock to a stand-alone processing step downstream. Among potential downstream processing steps, there can be mentioned aromatics saturation, desulfurization and denitrogenation, catalytic dewaxing, thermal cracking, and the like. In such a manner, VGO feedstocks have been selectively refined into gasoline, middle distillate and/or lube oil products having improved properties for sulfur, nitrogen and aromatics content, low temperature viscosity, burn temperature, etc.
Hibbs et al., "Alternative Hydrocracking Applications," published by UOP of Des Plaines, Ill. (1990), describes several processes wherein VGO feedstocks are initially hydrocracked under mild or high pressure conditions to produce a high quality, partially converted feedstock. Such feedstocks are used in a downstream thermal cracking unit for maximizing diesel output, an FCC unit for maximizing gasoline output, a catalytic dewaxing unit for enhancing a lube basestock and a steam cracker for producing ethylene.
Donnelly et al., Oil & Gas Journal, Oct. 27, 1980, pp. 77-82 describes a catalytic dewax process wherein wax molecules of a waxy gas oil are selectively cracked and the dewaxer effluent is fed to a stripper. A downstream hydrodesulfurization reactor can be placed either prior to or after the stripper.
Gembicki et al., Oil & Gas Journal, Feb. 21, 1983, pp. 116-128 describes a VGO conversion process wherein a hydrodesulfurizer or FCC feed hydrotreater is retrofitted as a mild hydrocracker (MHC) to increase middle distillate production.
S. L. Lee et al., "Aromatics Reduction and Cetane Improvement of Diesel Fuels," published by Akzo Chemicals NV, describes single-and two-stage processes for aromatics reduction and cetane improvement of diesel fuels. The single stage process consists of severe hydrotreatment of heavy diesel type feeds using a high activity NiMo catalyst. The dual stage process combines deep hydrotreatment pretreating of a light diesel-type feed to effect hydrodesulfurization and hydrodenitrogenation followed by hydrogenation over a noble metal catalyst.
U.S. Pat. No. 5,114,562 to Haun et al. describes the two-stage hydrotreatment of a middle distillate feed wherein the stream is hydrodesulfurized prior to hydrogenation over a noble metal catalyst. Following hydrotreatment, the feed is directed to a product recovery fractionation means.
U.S. Pat. No. 4,973,396 to Markey describes the two stage hydrotreatment of a virgin naphtha feed. Following a low pressure hydrotreater stage, the effluent is scrubbed and stripped of H.sub.2 S, and the stripper bottoms are fractionated into overhead and bottoms streams. The overhead stream is then hydrocracked using a noble metal catalyst, and the bottoms stream is fed to a product fractionator.
U.S. Pat. No. 4,990,242 to Louie et al. describes a process for producing low sulfur fuels wherein a virgin naphtha stream is fed to a first stage fractionator to produce overhead and bottoms streams. Both streams are then fed to parallel hydrotreatment units made up of a hydrotreater, an H.sub.2 S scrubber and a steam stripper. Effluents from the parallel strippers can be recombined for feed to a second stage fractionator.
U.S. Pat. No. 2,853,439 to Ernst, Jr. describes a combination distillation and hydrocarbon conversion process wherein a gas oil-type feed removed from a first fractionator is fed to a catalytic cracking reactor. A major portion of the cracked effluent is returned to a lower end of the first fractionator as a stripping stream. A minor portion of the cracked effluent is fed to a second fractionator. Overheads from the second fractionator are fed to an upper end of the first fractionator.
U.S. Pat. No. 3,671,419 to Ireland et al. describes a crude oil upgrading process wherein a VGO-type feed is hydrogenated, and the hydrogenator effluent is fractionated into overhead and bottoms streams. The fractionator overhead stream is fed to a hydrocracker and the fractionator bottoms stream is fed to a catalytic cracker. The cracked effluents are then fractionated into product streams.
As far as Applicants are aware, there is no previously known conversion process for producing upgraded middle distillate wherein the hydrocarbon feed is hydrocracked at moderate conditions, the hydrocracked effluent is cooled and fed to a product fractionator, a fractionator middle distillate sidedraw is first heated by heat exchange against the hydrocracker effluent stream and then introduced to a hydrotreater reactor, and the hydrotreater effluent is fed to a distillate side-stripper.